FIG. 4 is a diagram showing an electrical configuration of a power supply apparatus 100 provided with a conventional overcurrent detection circuit. The source electrode of a P-channel (P-type semiconductor) MOS transistor (insulated gate field-effect transistor) 101 is fed with an input voltage Va, and the drain electrode thereof is connected to the cathode of a diode 105 and to an input side of an overcurrent detection circuit 104 and, via an inductor 106, to one end of a capacitor 107 and to one end of a load 102. The anode of the diode 105, the other end of the capacitor 107, and the other end of the load 102 are grounded.
The overcurrent detection circuit 104 compares a voltage at the drain electrode of the MOS transistor 101 with a reference voltage thereof for monitoring whether or not the drain current of the MOS transistor 101 is an overcurrent (whether or not the drain current of the MOS transistor 101 is equal to or greater than an overcurrent detection level determined by the reference voltage), and gives the comparison result to a control portion 103. The control portion 103 monitors an output voltage VL to be applied to the load 102, and controls a voltage at the gate electrode of the MOS transistor 101 so that the output voltage VL is kept constant. The control portion 103 also receives an output of the overcurrent detection circuit 104, and thereby recognizes an overcurrent state of the MOS transistor 101. The input voltage Va is also fed to the control circuit 103 and the overcurrent detection circuit 104 as a power supply voltage thereof.
For protecting the power supply apparatus 100 from an overcurrent, there have been proposed three major methods (first to third methods, which will be described below). In the first method, once an overcurrent is detected, then the MOS transistor 101 is kept off. To cancel the off state, it is necessary to stop the supply of the input voltage Va and then restart the supply thereof.
However, the first method described above has the disadvantage that, if the overcurrent protection function operates as a result of a relatively high current (an inrush current) passing through the power supply apparatus 100 at start-up, the output voltage VL becomes 0 V, and therefore the power supply apparatus 100 fails to start (start-up failure occurs). Such start-up failure occurs notably in a case where the load 102 has a large input capacitance.
Assume that, to prevent such start-up failure, the overcurrent detection level set in the overcurrent detection circuit 104 is so set as to be greater than the maximum value of the inrush current. Then, even if a current nearly equal to the overcurrent detection level (ultimately, a current equal to the overcurrent detection level) continues to pass through the MOS transistor 101 due to variations in the load 102, for example, the MOS transistor 101 is kept on. This unfavorably leads to damage (damage by heat) not only of the MOS transistor 101 but also of the diode 105, the inductor 106, and the load 102, for example, and thus reduces the reliability of the power supply apparatus 100. On the other hand, to prevent damage to these components, as the power MOS transistor 101, the diode 105, the inductor 106 (in some cases, the load 102), or the like, those with larger rated currents have to be adopted. This leads to a larger mounting area, and also to a higher cost.
The second method is called a constant current drooping method, in which the drain current of the MOS transistor 101 is controlled so as not to exceed a certain level (i.e. the overcurrent detection level) irrespective of the value of the output voltage VL. In the second method, assume that the load 102 is short-circuited. Then, the drain current of the MOS transistor 101 is kept at the overcurrent detection level (or kept below the overcurrent detection level) without keeping the off state of the MOS transistor 101. Adopting the second method described above can prevent start-up failure as observed in the first method.
In the third method, an operation mode is switched according to a predetermined time as described in Patent Document 1 listed below. Disclosed is a power supply apparatus provided with an overcurrent protection function wherein, upon receiving from a switching control portion a power supply start signal from a remote ON/OFF terminal, a main switching portion starts to operate, a current of the main switching portion is detected, and, if an overcurrent is detected, an overcurrent protection circuit operates so as to control the switching control portion. This power supply apparatus is provided with a timer circuit that, upon receipt of the power supply start signal, transmits an “overcurrent protection set value at the time of overload” within a timer set time t1 and outputs to the overcurrent protection circuit a signal for switching to a rated output, a “steady overcurrent protection set value” after the timer set time t1 is elapsed (see, for example, Patent Document 1).
Patent Document 1: JP-A-08-065879